Exhaust gas catalyst and method of manufacturing same

ABSTRACT

The present invention involves an exhaust gas catalyst and method of manufacturing same. The invention provides for a cost-effective material which lowers the cold-start emissions from the exhaust of vehicles. The invention is a passive system which accelerates the light-off temperature of catalyst in a cost-effective fashion. The invention includes a method of manufacturing an exhaust gas catalyst capable of lowering cold-start emissions including the steps of providing an oxide mixture having praseodymium and cerium, doping about 0-10% weight zirconium and about 0-10% weight yttrium to the oxide mixture, adding about 0-2% weight metal including palladium, platinum, or rhodium to the oxide mixture, mixing gamma aluminum to the oxide mixture for washcoating and washcoating the oxide mixture onto a monolithic substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/511,592, filed Feb. 23, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to an exhaust gas catalyst capable oflowering cold-start emissions and a method of manufacturing the exhaustgas catalyst.

[0004] 2. Background Art

[0005] The need to lower vehicle exhaust emissions continues to createchallenges, especially in the wake of stringent environmentalregulations. The need to lower cold-start emissions is at the heart ofmost emissions reduction strategy, since cold-start emissions accountfor a significant portion of exhaust emissions from any given vehicle.During startup, hydrocarbons can be passed through the exhaust systembefore the catalyst can heat up and convert the hydrocarbons to moredesirable gases. Although a large portion of hydrocarbons are reduced,an amount of hydrocarbons and other undesirable gases may be allowed topass through the exhaust system without reduction thereof.

[0006] One solution to the cold-start emission problem is providing amaterial that is able to give off oxygen to the catalyst during startupconditions such that the “light-off” temperature of the catalyst isaccelerated. The light-off temperature is the temperature at which thecatalyst reacts with hydrocarbons and other exhaust gases to reducethese gases, which are undesirable, to more desirable gases such ascarbon dioxide. Oxygen, when fed to the catalyst, creates an exothermicreaction to the catalyst, resulting in increased temperature whichallows the catalyst to reach the light-off temperature more quickly.

[0007] Currently, palladium is used with a cerium-zirconium mixed oxidesupport, an aluminum oxide support, or a mixture thereof to give offoxygen at startup conditions (low temperature), in order to acceleratelight-off of the catalyst. However, palladium is an extremely expensivematerial which typically contributes to approximately 95% of the totalcost of the catalyst. Recent studies have thus focused on methods andmaterials to reduce the consumption of palladium while providing aneffective means for accelerating the light-off temperature of thecatalyst.

[0008] The use of additional hardware has also been proposed to solvethe cold-start problem. In some cases, the additional hardware allowsthe exhaust system catalyst to be heated directly in order to acceleratelight-off of the catalyst. For example, an electrically heated catalystmay be used where the catalyst is heated directly by an electric heater.Prior to startup, current is run through the heater via the vehiclebattery, and the heat produced accelerates light-off of the catalystand, in turn, lowers the cold-start emissions.

[0009] Another example of an active approach is to allow fuel to combustnear the catalyst to quickly raise the temperature thereof. This isaccomplished by running lines and having an ignition system disposed onthe vehicle. Generally, systems that incorporate additional hardware,such as the examples mentioned above, result in high costs forimplementation.

[0010] Thus, what is needed is a cost effective solution to thecold-start emissions problem.

DISCLOSURE OF INVENTION

[0011] Accordingly it is an object of the present invention to provide acost effective material which lowers the cold-start emissions from theexhaust of vehicles.

[0012] It is another object of the present invention to provide apassive system which accelerates the light-off temperature of catalystsin a cost-effective fashion.

[0013] Yet another object of this invention is to provide a method ofmanufacturing an exhaust gas catalyst capable of lowering cold-startemissions through a three-way catalyst including mixed metal oxides.

[0014] The method provides an exhaust gas catalyst having an oxidemixture with substantially equal molar content of praseodymium andcerium, doping about 0 to 10 weight % zirconium and about 0-10 weight %yttrium to the oxide mixture by combining suitable precursors in aliquid solution before oxide formation, and adding or doping about 0-2weight % precious metal including palladium, platinum, or rhodium to theoxide mixture. The method further involves mixing gamma alumina to theoxide mixture for washcoating, and washcoating the mixture onto amonolithic substrate.

[0015] Another specific object of this invention is an exhaust gascatalyst supported on a monolithic substrate in which the catalyst iscapable of lowering cold-start emissions. The catalyst comprises anoxide mixture which is formed by combining suitable precursors ofpraseodymium, cerium, and at least one metal selected from the groupconsisting of zirconium and yttrium, in a liquid solution before oxideformation, and which is washcoated on the monolithic substrate. Theoxide mixture has substantially equal molar content of praseodymium andcerium. The oxide mixture also has about 0 to 10 weight % zirconium andabout 0-10 weight % yttrium, and about 0-2 weight % precious metal whichincludes palladium, platinum, or rhodium.

[0016] In another object of this invention provides for a method ofmanufacturing an exhaust gas catalyst capable of lowering cold-startemissions through a three-way catalyst including mixed metal oxides.

[0017] The method provides an exhaust gas catalyst having an oxidemixture of praseodymium and cerium, doping about 0 to 10 weight %zirconium, about 0-10 weight % yttrium to the oxide mixture, and about0-2 weight % precious metal including palladium, platinum, or rhodium tothe oxide mixture. The method further involves mixing gamma alumina tothe oxide mixture for washcoating, and washcoating the mixture onto amonolithic substrate.

[0018] In yet another embodiment of the invention, a method of making anoxygen storage material for automotive exhaust catalysts is providedwherein oxygen storage materials having a low temperature of oxygenrelease, such as praseodymia can be made thermally stable by anorganic-templating method that incorporates low levels of zirconia,yttria and possibly other additives.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 is a table which shows the effect of surface area of mixedoxides as concentration of zirconium is increased in an equal molarconcentration mixture of praseodymia and ceria;

[0020]FIG. 2 is another table which shows the oxygen storage capacitiesof mixed oxides as the concentration of zirconium is increased in anequal molar concentration mixture of praseodymia and ceria;

[0021]FIG. 3 is a graph which shows the oxygen storage capacities ofmixed oxides having praseodymia verus mixed oxides without praseodymia;

[0022]FIG. 4 is a bar graph which shows the oxygen storage capacities ofmixed oxides having praseodymia and adding precious metal at the timethe mixed oxide is made versus mixed oxides without praseodymia;

[0023]FIG. 5 is a graph which shows the average catalyst temperature asa function of time following cold start of mixed oxides havingpraseodymia and ceria in approximately equal molar concentration; and

[0024]FIG. 6 is a graph which shows how the increased catalysttemperature (of the mixed oxides used in FIG. 5) corresponds toincreased conversion of reductants in the exhaust gas, as shown fortotal hydrocarbons (THC).

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] The invention is a method for manufacturing an exhaust gascatalyst capable of lowering cold-start emission. More specifically, themethod teaches the formation of high surface area, thermally stableoxygen storage materials for automotive exhaust catalysts. The methodgenerally includes providing an oxide mixture having praseodymium andcerium, doping about 0-10 weight % zirconium and about 0-10 weight %yttrium to the oxide mixture, and adding about 0-2 weight % preciousmetal to the oxide mixture. The precious metal may include palladium,platinum, rhodium or mixture thereof. The method generally furtherincludes mixing gamma alumina to the oxide mixture for washcoating andwashcoating the mixture onto a monolithic substrate. As a result, amaterial may be manufactured that lowers cold-start emissions. Theresulting material is an exhaust gas catalyst supported on a monolithicsubstrate in which the catalyst comprises a praseodymium-cerium oxidemixture washcoated on the monolithic substrate.

[0026] Throughout the specification, the term “oxide mixture” or “mixedoxide” refers to a solid solution mixed oxide, or alloy, rather than aphysical mixture of single oxides.

[0027] A surprising and unexpected advantage of the present method isthat the oxide mixture so produced has been found to have high surfacearea and high oxygen storage capacities at lower temperatures whileconsuming lower contents of metals such as palladium. As describedbelow, the presence of zirconium with the praseodymium increases surfacearea stability of the oxide mixture. Such advantage provides forsignificant cost savings in manufacturing exhaust gas catalyst capableof lowering cold-start emissions through an exhaust stable catalyst,such as a three-way catalyst. The resultant praseodymium-cerium oxidemixture requires less metals to be added thereto in order to acceleratelight-off of the catalyst. Thus, total consumption of palladium,platinum or rhodium will be reduced with implementation of the presentinvention method.

[0028] According to the present invention, an oxide mixture is providedhaving preferably substantially equal molar content of praseodymium andcerium. Alternatively, the oxide mixture may be of different molarcontents of praseodymium and cerium, e.g., about 40%/60%, 60%/40%,respectively. The oxide mixture may be prepared by any suitable means.Preferably, the oxide mixture is prepared as described in U.S. patentapplication Ser. No. 08/966,729, entitled “Thermally Stable,High-Surface-Area Metal Oxides Made By Organic Templating”, now U.S.Pat. No. 6,139,814, which is hereby incorporated by reference.Specifically, the oxide mixture is preferably prepared using a uniquetemplating method, where the template is a porous structured organicmaterial with a well-defined crystallinity or microcrystallinity. By“structured” is meant a well-defined pore structure which optimally, isrelatively homogeneous throughout the organic material. It is believedthat the method works to produce a porous metal oxide somewhat like acast is made from a mold. That is, the organic template serves as themold and the porosity of the template material helps determine theultimate structure of the resulting metal oxide. The mold (i.e., poresof the organic template) is contacted with a liquid solution of metalsalts or organometallic compounds (hereinafter “metal oxide precursors”)that optimally essentially fill the pores, acting as the cast material.Subsequent heating then vaporizes and removes the liquid, leaving themetal oxide precursors adsorbed on the surface of the mold (i.e., theporous organic template material), and then further heating in air orsome other gas containing oxygen at high temperatures combusts the moldwhile also converting the metal oxide precursors to their respectivemetal oxides. However other suitable ways of preparing the oxide mixturemay be used, such as sol gel methods or precipitation methods do notfall beyond the scope and spirit of this invention.

[0029] The oxide mixture is then doped with zirconium, yttrium, or bothto enhance the thermal stability of the high surface area oxide mixture.

[0030] Generally, between 0 to about 10 weight % zirconium and between 0to about 10 weight % yttrium may be doped to the oxide mixture. Asdescribed below, it has been found that higher doses of zirconium mayreduce the oxygen storage capacities of the oxide mixture. It ispreferred that the least amount of zirconium be doped to the oxidemixture that achieves the desired characteristics.

[0031] In a preferred embodiment, 0-2 weight % zirconium and/or yttriumis doped to the oxide mixture. More specifically, it has been found thatthe addition of zirconia tends to promote an undesirable transformationof tetravalent praseodymia (Pr+4) to trivalent praseodymia (Pr+3) in thepraseodymia-ceria mixed oxide. This was confirmed by thermal gravimetricanalysis which showed that the amount of oxygen reversibility releasedupon heating from 200°to 800°celsius in air decreases with increasinglevels of zirconia. Thus, only small portions of zirconium arepreferably added in order to enhance stability of the surface area ofthe mixed oxides.

[0032] As shown in FIG. 1, oxide mixtures having substantially equalmolar content of praseodymium and cerium increase in surface area as thelevels of zirconium are increased in the mixtures thereof. Moreover,FIG. 1 shows that increased contents of zirconium in oxide mixtureshaving equal molar contents of praseodymium and cerium increase thesurface area of the resulting oxide mixture for fresh or air aged oxidemixtures. The increase in surface area of the oxide mixtures withrespect to zirconium illustrates their enhanced ability as catalystsupport.

[0033] As shown in FIG. 2, increased levels of zirconium in oxidemixtures having equal molar content of praseodymium and cerium result ina decrease of the oxygen storage capacities of the resulting oxidemixtures. Moreover, FIG. 2 shows that as levels of zirconium increase inoxide mixtures, oxygen storage capacities of such oxide mixtures alsodecrease consistent with temperature. The decrease of oxygen storagecapacities of the oxide mixtures illustrates an adverse effect ofzirconium on catalyst support. Additionally, FIGS. 1 and 2 togetherillustrate the need to optimize the amounts of zirconium doped to theoxide mixture.

[0034] As shown in FIG. 3, resulting oxide mixtures having praseodymiumat 100° to 250° celsius have typically higher oxygen storage capacitiesthan the resulting oxide mixture without praseodymium contents.Additionally, FIG. 3 also shows that oxide mixtures having greatercontents of zirconium typically have a lower oxygen storage capacitythan oxide mixtures having less zirconium contents.

[0035] Doping of the oxide mixture with zirconium, yttrium, or both maybe achieved according to the preparation of preparing oxide mixtures asdescribed in our U.S. Pat. No. 6,139,814. Thus, according to the presentinvention, “doping” of a praseodymia-ceria oxide mixture with zirconium,yttrium, or both, is preferably achieved by dissolving the selectedmetal oxide precursors in solution. It will be appreciated that themetal oxide precursors may be selected from any precursor which willprovide a metal or a metal compound on the templating material, whichmetal or compound is capable of converting to its metal oxide during thesubsequent heating of the template material. The metals can be suppliedvia any salts that are soluble in the solvent. Nitrates are particularlysuitable since they generally show good solubility in aqueous solution.Other common precursors which have been successfully used includeorganic salts such as citrates and acetylacetonates of the metal. By wayof example only, where the oxide mixture is to be “doped” withzirconium, the precursors may be inorganic metal salts or organic metalcomplexes like zirconium nitrate, zirconium (IV) acetylacetonate, andzirconium (IV) citrate. Still other metal oxide precursors will beapparent to those skilled in the art in view of the present disclosure.

[0036] As we teach in our U.S. Pat. No. 6,139,814, the solvent used inpreparing the desired liquid solution of metal oxide precursors can beany liquid which is capable of both dissolving the metal precursors andbeing absorbed by the organic template material. Although organicsolvents, such as alcohols, ethers, and ketones can be used, water isthe most convenient and preferred solvent. Even in cases where the metalprecursors have limited solubility in water, it is often possible toincrease solubility through the addition of citric acid to the aqueoussolution of metal salts. The citric acid reacts with the metal salts toform citrate complexes which have a high solubility in water. Thisallows the preparation of mixed oxide materials from more highlyconcentrated solutions of metal salts than would otherwise be possible.Another method for improving the solubility of precursor salts is toheat the solution to between 50-100° C., preferably to about 75-80° C.It should also be noted that the technique works best in either acidicor neutral mediums.

[0037] After the oxide mixture is doped, precious metal is added to theoxide mixture. Generally, 0-2 weight % precious metal is added to theoxide mixture. Specifically, the precious metals which may be addedinclude palladium, platinum or rhodium. Preferably, 0.1 to 1.5 weight %of metal may be added to the oxide mixture. Moreover, a combination ofthe three precious metals may be added at any suitable ratios. Additionof the precious metal may be performed by impregnating the oxide mixturetherewith, as known in the art.

[0038] It has been found that if precious metal is added to the mixedoxide at the time the mixed oxide is made, i.e., with the precious metalprecursors being added to the liquid solution of metal oxide precursorsprior to templating, then oxygen storage capacities and surface areas ofthe resulting solid solution oxide mixture are typically higher thanwhen precious metal is added at a time after the oxide is made. Thus, itis preferred that the precious metal be added to the oxide mixture atthe time the oxide mixture is prepared and doped. However, it is to benoted that adding precious metal at a later time does not fall beyondthe scope and spirit of this invention.

[0039]FIG. 4 illustrates that by providing an oxide mixture havingpraseodymium and by adding precious metals to the mixed oxide at thetime the oxide mixture is made, a catalyst with higher oxygen storagecapacity at low temperatures is obtained. As shown in FIG. 4, a catalysthaving praseodymium has a higher oxygen storage capacity than catalystswithout praseodymium. In turn, less precious metals, such as palladium,are required to obtain a given oxygen storage capacity. A decrease incontent of palladium per unit weight of catalyst results in asubstantial cost savings in manufacturing exhaust gas catalysts.

[0040] After the precious metal is added, a binder may be used such aszirconia, nitrates of ceria or praseodymia, and possibly gamma alumina.The binder would be mixed with the oxide mixture for washcoating. Theuse of a binder aids in bonding the contents to achieve a monolithiccatalyst having increased oxygen storage capacities. As an example,gamma alumina may be mixed with the oxide mixture at about 0.1/1 to 1/1in gamma alumina/oxide mixture molar ratios. It should be noted that abinder is not required to achieve the improved catalytic device of thisinvention. If a binder is used, the preferred binders are zirconia andnitrates of ceria and praseodymia. It is further believed that whengamma alumina is used as a binder for the present invention, gammaalumina converts into an aluminate-reducing the oxygen storage capacityof the catalyst.

[0041] As known, there are a number of ways to mix a binder with theoxide mixture for washcoating. All of such known methods may be used inthis invention for such purpose and do not fall beyond the scope andspirit of this invention.

[0042] The resulting oxide mixture may then be washcoated onto amonolithic substrate. As known, there are a number of ways to washcoatthe mixture onto the monolithic substrate. Any such methods may be usedfor this purpose and do not fall beyond the scope and spirit of thisinvention. Among the list of known methods, there is included dippingthe washcoat into a slurry containing the oxide mixture, and blowing theresulting oxide mixture onto the washcoat, etc.

[0043] The substrate typically may be a substrate including cordieritesubstrate or metallic substrate. The additional weight of the resultingoxide mixture to be washcoated onto the substrate is about 30%-50% ofthe weight of the initial substrate.

EXAMPLE 1

[0044] Praseodymium-cerium oxide mixture is prepared at equally molarcontents, specifically 45 weight percent each. Ten weight percentzirconium is doped to the oxide mixture and 2 weight percent palladiumwas added thereafter. Gamma alumina is then mixed to the oxide mixturefor washcoating and the oxide mixture is then washcoated onto amonolithic substrate, specifically cordierite substrate.

EXAMPLE 2

[0045] Praseodymium-cerium oxide mixture is prepared at equally molarcontents, specifically 42.5 weight percent each. Fifteen weight percentzirconium was doped to the oxide mixture and 2 weight percent palladiumis added thereafter. Gamma alumina is then mixed to the oxide mixturefor washcoating and the oxide mixture is then washcoated onto amonolithic substrate, specifically cordierite substrate.

EXAMPLE 3

[0046] Praseodymium-cerium oxide mixture is prepared at equally molarcontents, specifically 40 weight percent each. Twenty weight percentzirconium is doped to the oxide mixture and 2 weight percent palladiumwas added thereafter. Gamma alumina is then mixed to the oxide mixturefor washcoating and the oxide mixture is then washcoated onto amonolithic substrate, specifically cordierite substrate.

EXAMPLE 4

[0047] Praseodymium-cerium oxide mixture is prepared at equally molarcontents, specifically 31 weight percent each. Thirty-eight weightpercent zirconium is doped to the oxide mixture and 2 weight percentpalladium was added thereafter. Gamma alumina is then mixed to the oxidemixture for washcoating and the oxide mixture is then washcoated onto amonolithic substrate, specifically cordierite substrate.

EXAMPLE 5

[0048] A catalyst, made by washcoating a cordierite monolithic supportwith Pd loaded onto a solid solution praseodymium-cerium oxide mixturecontaining approximately equal amounts of praseodymium and cerium, wasincorporated into a vehicle exhaust system in a close-coupled positionapproximately 18 inches from the engine exhaust manifold.

[0049] Emissions data, together with the catalyst temperature, wereacquired during dynamometer tests performed according to the standardFederal Test Procedure, FTP-75. Additionally, as shown in FIG. 5, inorder to demonstrate the ability of the catalyst to store and releaseoxygen for the purpose of accelerating catalyst light-off, either air ornitrogen was passed over the catalyst during catalyst cool-down prior toeach test. Four tests were performed with each pretreatment. The averagecatalyst temperature for both air cooling and nitrogen cooling, togetherwith vehicle speed, are plotted as a function of time following coldstart in FIG. 5. It is apparent that between approximately 18 and 35seconds into the test, the catalyst temperature is higher following aircooling than nitrogen cooling. The increased catalyst temperaturecorresponds to increased conversion of reductants in the exhaust gas, asshown for total hydrocarbons (THC) from a pair of tests in FIG. 6.Consequently, the THC emissions collected in Bag 1 for FTP-75 (whichincludes cold-start) are lower for the air than the nitrogenpretreatments as shown in Table 1 below, which compares the averages ofall four tests. TABLE 1 Bag 1 emissions of THC in grams/mile. Airpretreatment Nitrogen pretreatment 0.109 0.115

[0050] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed:
 1. A method of making a washcoat for use in themanufacture of a catalytic device, the method comprising: preparing aliquid solution including metal oxide precursors of praseodymium andcerium, and at least one additional metal selected from the groupconsisting of zirconium and yttrium; absorbing the liquid solution ontoa porous structured organic templating material that is capable of beingcombusted at an elevated temperature; heating the organic templatingmaterial containing the absorbed liquid solution to vaporize the liquid,convert the precursors to metal oxides, and combust said organictemplating material, whereby a solid solution praseodymium-cerium oxidemixture doped with at least one of zirconium and yttrium is obtained;and adding a precious metal to the oxide mixture in an amount no greaterthan about 2 wt. %.
 2. The method of claim 1, wherein the relative molarcontent of praseodymium to cerium is in the range of about 0.67:1 toabout 1.5:1.
 3. The method of claim 1, wherein the oxide mixture haspraseodymium and cerium in substantially equal molar content.
 4. Themethod of claim 1, wherein the oxide mixture includes zirconium in anamount no greater than about 10 wt. %.
 5. The method of claim 1, whereinthe oxide mixture includes yttrium in an amount no greater than about 10wt. %.
 6. The method of claim 1, wherein the precious metal is selectedfrom the group consisting of platinum, rhodium, palladium, and mixturesthereof.
 7. The method of claim 6, wherein the precious metal is addedin an amount in the range of about 0.1-1.5 wt. %.
 8. The method of claim1, wherein adding the precious metal includes mixing an aqueousprecursor of the precious metal into the liquid solution beforeabsorbing.
 9. A method of making a washcoat constituent for use in themanufacture of a catalytic device, the method comprising: forming asolid solution oxide mixture from an aqueous solution including metaloxide precursors of praseodymium, cerium, and at least one metalselected from the group consisting of zirconium and yttrium, wherein therelative molar content of praseodymium to cerium is in the range ofabout 0.67:1 to about 1.5:1 the oxide mixture; and adding, to the oxidemixture, a precious metal selected from the group consisting ofplatinum, rhodium, palladium, and mixtures thereof, in an amount nogreater than about 2 wt. %.
 10. The method of claim 9, wherein the oxidemixture has praseodymium and cerium in substantially equal molarcontent.
 11. The method of claim 9, wherein the oxide mixture includeszirconium in an amount no greater than about 10 wt. %.
 12. The method ofclaim 11, wherein the precious metal is added in an amount in the rangeof about 0.1-1.5 wt. %.
 13. The method of claim 9, wherein the oxidemixture includes yttrium in an amount no greater than about 10 wt. %.14. The method of claim 13, wherein the precious metal is added in anamount in the range of about 0.1-1.5 wt.
 15. The method of claim 91,wherein adding the precious metal includes mixing an aqueous precursorof the precious metal into the liquid solution.
 16. An exhaust gascatalyst supported on a monolithic substrate, the catalyst comprising: asolid solution oxide mixture formed from an aqueous solution includingmetal oxide precursors of praseodymium, cerium, and at least one metalselected from the group consisting of zirconium and yttrium, wherein therelative molar content of praseodymium to cerium is in the range ofabout 0.67:1 to about 1.5:1 the oxide mixture, and a precious metaladded to the oxide mixture in an amount no greater than about 2 wt. %,the precious metal being selected from the group consisting of platinum,rhodium, palladium, and mixtures thereof.
 17. The catalyst of claim 16,wherein the oxide mixture has praseodymium and cerium in substantiallyequal molar content, and zirconium in an amount no greater than about 10wt. %.
 18. The catalyst of claim 16, wherein the oxide mixture includesyttrium in an amount no greater than about 10 wt. %.
 19. The catalyst ofclaim 16, wherein adding the precious metal includes mixing an aqueousprecursor of the precious metal into the liquid solution.
 20. Thecatalyst of claim 19, wherein the precious metal is added in an amountin the range of about 0.1-1.5 wt. %.
 21. An exhaust gas catalystsupported on a monolithic substrate, the catalyst comprising: a solidsolution oxide mixture formed from an aqueous solution including metaloxide precursors of praseodymium, cerium, at least one metal selectedfrom the group consisting of zirconium and yttrium, and a precious metalin an amount no greater than about 2 wt. %, wherein the relative molarcontent of praseodymium to cerium is in the range of about 0.67:1 toabout 1.5:1 the oxide mixture.
 22. The catalyst of claim 21, wherein theprecious metal is selected from the group consisting of platinum,rhodium, palladium, and mixtures thereof.